The question of the day seems to involve the difference between the original cathedral-port and modern rectangular-port LS heads. More specifically, which one is better? Were we to compare two sets of production heads, there would be little comparison, as even the very best production (cathedral-port) LS6 heads (243 castings) offer nowhere near the flow and power potential of the current LS3 heads. Based on flow numbers alone, the LS3 head has the LS6 heads covered by 50-60 cfm on the intake side (though the change in chamber volume will drop the static compression ratio). The flow figures alone make the swap over to the LS3 components very desirable, especially considering the fact that the stock LS3 head will support well over 600 horsepower (we recently made 690 hp on a 468 stroker with stock LS3 heads). The downside is that the head swap from LS1/LS6 heads to LS3 heads is more involved than a simple R&R. The new heads require a 4.00-inch bore block, LS3 rockers and intake manifold as well, both increasing the cost and limiting the swap potential to 6.0-6.2L motors (and big-bore stroker variants).
Measured stock to stock, LS3 heads will offer power gains over the cathedral-port heads, but what about the aftermarket stuff? After the introduction of the LS3 heads, many enthusiasts started looking down their noses at the original cathedral-port heads, obviously not remembering how much better they were than the original small-block heads they replaced. Has the advent of the rectangular-port LS3 heads relegated the cathedral-port heads to second-tier status or is this just a case of bad PR? To answer this question, we enlisted the aid of Mast Motorsports. Included in their listing of performance components for the LS engine family was an impressive array of both cathedral and rectangular-port cylinder heads. In fact, they had the ideal set of heads for our comparison. One of our concerns for the head test was addressing the potential change in static compression ratio. Traditionally, cathedral-port heads offer smaller chambers than their rectangular-port cousins. The change in compression ratio can be as much as a full point or more, which can improve power by as much as 3-4 percent (near 25 hp on a 600hp motor).
To cure this issue, we selected a set of cathedral-port heads with 70cc combustion chambers to match the chamber volume typical in rectangular-port heads. Many aftermarket manufacturers are now offering larger chambers on their performance cathedral-port heads, as many find their way onto larger displacement strokers. Equalizing the compression ratio eliminated the variable, so we could concentrate on the power difference associated with the flow numbers, port size, and efficiency. For our test, Mast sent us a set of their cathedral-port, 11-degree, six-bolt, medium-bore CNC heads along with a set of LS3 heads that also featured an 11-degree valve angle and full CNC porting. In the tale of the tape, the cathedral-port heads offered 245cc intake ports, a 2.08/1.60 valve combination and peak flow numbers of 338 cfm on the intake and 237 cfm on the exhaust. The CNC-ported LS3 heads stepped up these numbers with an intake port volume of 256cc, a 2.165/1.60 valve package, and peak flow numbers of 372 cfm on the intake and 261 cfm on the exhaust. On numbers alone (like their production counterparts), the ported Mast LS3 heads seemed to hold a clear advantage over the cathedral-ports.
Another area of concern when it comes to any comparison between the cathedral and rectangular-port heads is cam timing. Optimum cam timing is a function of effective operating range, but another important factor is the relationship between the intake and exhaust flow. Typically, the superior intake flow offered by the LS3 head lowers the intake to exhaust flow relationship. This can be equalized (or optimized) by increasing the exhaust duration on the cam relative to the intake. On our Mast heads, the LS3 head offered both more intake and exhaust flow than the cathedral-port heads, resulting in an intake-to-exhaust flow percentage (372/261 cfm) of 69.89 (meaning the exhaust flowed roughly 70 percent of the intake). By comparison, the Mast cathedral-port head checked in slightly higher at 70.1 percent (338/237 cfm). Typically we'd see more of a difference between the two, but the intake-to-exhaust flow differed by less than 1 percent (actually 3/10 ths of a point). Despite the similarity, we decided to run two different cam profiles with the heads. Before you cry foul, know that we ran both heads with both cams just to see how each would respond.
Running a pair of heads with a pair of cams meant we had to double up on cam swaps, but it was all in the name of science, so we didn't mind putting in the extra work. For cam choices, we went right to the Comp Cams catalog and selected cam profiles designed specifically for the cathedral and rectangular-port heads. On the cathedral-port side, we chose a 289LRHR14 (PN 54-461-11) that offered .624-inch lift, a 239/247 duration split, and a 114-degree lobe separation angle. The 289LRRHR14 rectangular-port cam offered the same .624-inch lift, 239-degree intake duration and 114-degree LSA, but increased the exhaust duration to 255 degrees. LS3 cams typically offer a wider spread between the intake and exhaust than comparable LS1 cams. The idea is to help the (relatively) limited exhaust flow with additional exhaust duration. Not only would we show the difference between the two head configurations, but also the difference with their respective cams. How would the cathedral-port heads work with the rectangular-port cam and vice versa? Questions like these are what keep us up at night and why we do so much testing here at GM High-Tech.
Obviously our high-flow heads required something other than a stock 6.0L or even an LS3 short-block. Knowing the heads will support over 600 hp, we built our test motor accordingly. The 6.0L iron block was bored .030 over then treated to a forged 4.0-inch stroker crank and rods from ProComp Motorosports along with a set of dished pistons from Probe Racing.
The slugs were combined with a ring set from Total Seal and a precision balance job from the boys at L&R Automotive. The combination has already exceeded 1,000 hp in boosted form, so we knew it was plenty stout and more than sufficient for our head test. The short-block was assembled first with the Comp cathedral-port cam (and new hydraulic roller lifters), FAST LSXr intake and matching 102mm throttle body. FAST also supplied the fuel rail and 75-pound injectors for our test along with an XFI engine management system. Additional components used on the motor included a complete Moroso oiling system, Fel Pro MLS head gaskets, and American Racing 1-7/8-inch headers. Equipped as such, the cathedral-port heads and cam combo pumped out 631 hp and 575 lb-ft of torque, with torque production exceeding 550 lb-ft from 4,200 rpm to 5,800 rpm.
Having dialed in the combination, we proceeded to swap out the cam. After installation of the rectangular-port cam, the peak power numbers improved slightly to 639 hp and 578 lb-ft. The increased exhaust duration offered by the rectangular-port cam (255 degrees versus 247 degrees) improved power production slightly from 5,500 rpm on up, but the improvements in top-end power came with a penalty. The increased exhaust duration had a negative effect on low-speed power, as power was down from 3,000 rpm to 4,700 rpm, the greatest difference of 20 lb-ft coming at 3,100 rpm. This should not come as a huge surprise, as increased duration usually follows this trend. Increased duration is done to increase the effective engine speed. The additional exhaust duration also decreased idle vacuum slightly compared to the cathedral-port cam, so low-speed drivability might also suffer (we never loaded the motor below 3,000 rpm).
Though the dyno test was run in reverse order, we will provide the data for the rectangular-port heads first with the cathedral-port cam. Swapping over to the Mast LS3 heads required a change in intake manifold as well. Once again we relied on a FAST LS3 LSXr intake and 102mm throttle body along with the 75-pound injectors, rails and XFI management. Given the difference in airflow, we were naturally excited about replacing the cathedral-port heads with the rectangular-port heads. Run with the Mast CNC LS3 heads and Comp cathedral-port cam, the 408 stroker produced 634 hp and 577 lb-ft of torque, meaning a difference of just 3 hp and 2 lb-ft measured peak to peak. In reality, the cathedral-port heads were within 1-2 hp at the peak, but offered as much as 20 additional lb-ft below 4,000 rpm. We suspected that the rectangular-port heads were not optimized with the cathedral-port cam, but we were still surprised to see such a small difference in power given the extra airflow offered by the LS3 heads.
The final test run was to combine the rectangular-port cam with the Mast LS3 heads. The combination produced the highest peak power numbers of the day (640 hp and 680 lb-ft), but only by 1 hp and 2 lb-ft over the cathedral-port head and rectangular-port cam combo. The LS3 heads did respond better to the installation of the rectangular-port cam. The cam improved power from 5,300 rpm on up, with only a slight loss in power below 3,300 rpm. Obviously there is something to increasing efficiency of the exhaust port with increased duration. Though we were happy with the results of the cam test, we were still surprised that the LS3 heads offered little or no power over their cathedral-port counterparts. Given the disparity in airflow, we expected to see 10-15 hp, but the results of this head-to-head shootout just go to show that airflow isn't everything. It also showed that you can't go wrong with either set of Mast heads on your performance LS combination.
Coefficient of DischargeThough this may be a tad on the technical side, it might well help explain why the cathedral-port heads did so well against the rectangular-port heads despite the difference in flow. The coefficient of discharge is essentially a measurement of the efficiency of the port (actually the curtain area of the valve). The curtain area of the valve is easy to visualize. Imagine dropping a circular shower curtain down from the outside diameter of the intake valve. The length of the shower curtain would be determined by the maximum valve lift, but (more importantly), the curtain area can be calculated at every valve lift where airflow is measured (usually .100-.700-inch lift). Much like average airflow through the head port, we can take the average coefficient of discharge by combining the averages for all the lift values and then dividing by the number of lift points. The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift. Applying the formula to our heads, we see that though the LS3 heads offer a better coefficient of discharge at the maximum lift of .700, the cathedral-port head offered not only a superior coefficient of discharge from .100-.400 lift, but a better average coefficient of lift from .100-.600 lift. Could it be that the efficiency of the port overcame the absolute flow of LS3 heads?